1. Field of the Invention
The field of this invention relates to a carbon deposition preventive double-layer tube which is used for handling carbonaceous compounds such as hydrocarbons, their derivatives, carbon monoxide, or the like at temperatures higher than about 500.degree. C.
2. Prior Art
Heretofore, tubes for handling carbonaceous compounds such as hydrocarbons, their derivatives, carbon monoxide, or the like have been generally made of steels or Ni alloys. Carbon deposition frequently occurs on protons such as heater tubes, piping, heat exchangers and the like, exposed to high temperature fluids containing carbon compounds. Carbon deposition brings about undesirable effects such as increased pressure drop, decreased heat transfer efficiency, and the like, whereby it is necessary to frequently perform decoking. Decoking is an obstacle to a steady operation, harming the economics of process and also brings about various disadvantages of the construction materials of the apparatus.
From the viewpoint of corrosion resistance, Cr is normally added to the materials of conventional tubes consisting of steels or Ni alloys. The amount of Cr added to these materials is less than 28% by weight, and heat resisting steels or alloys normally containing around 25% by weight of Cr. In the first stage of use, a protective oxide film such as Cr.sub.2 O.sub.3 film stays on the surfaces of the conventional tubes. Since normal operating conditions comprise a carburizing/oxidizing atmosphere with thermal cycles, the Cr just beneath the surface of the conventional tube is gradually consumed thereby to cause deterioration of the surface. In this specification, "a carburizing/oxidizing atmosphere" means an atmosphere in which some elements are carburized and other elements are oxidized depending on the carbon and oxygen potentials of the atmosphere. "The deterioration of the surface of the tube under a carburizing/oxidizing atmosphere" in the present specification mans a phenomenon that in the case that the protective oxide film is broken, carbon penetrates and diffuses into the interior of the tube wall from the surface, reacting with the Cr in the tube material alloy to consume the Cr and form Cr carbides, whereby the Cr content of the matrix becomes poor, and the surface can readily be oxidized, so that corrosion rapidly proceeds by carburization and oxidation of the surface. In this case, the protective oxide film such as Cr.sub.2 O.sub.3 is lost and oxide layers consisting essentially of Fe and Ni are formed instead. Iron oxides such as Fe.sub.2 O.sub.3, nickel oxides such as NiO, or spinel oxides such a NiFe.sub. O.sub.4, FeCr.sub.2 O.sub.4, and the like appear on the outer surface of the tube. The iron oxides and nickel oxides are easily reduced by carbonaceous compounds into iron and nickel metals which act as a catalyst to cause carbon deposition.
An apparatus for handling carbonaceous compounds, which is made of an Fe-based, a Ni-based, a Co-based, or their mixed based alloy which contains not less than 28% by weight of Cr was provided by the present applicants. The apparatus made of a Cr-rich alloy is described in Japanese Patent Application First Publication "Kokai" 63-31535. The Cr-rich alloy containing not less than 28% by weight of Cr has a lower strength than the conventional alloys such as 25Cr-20Ni cast steel or 25Cr-35Ni cast steel at elevated temperatures. Therefore, in practice, the wall thickness of the tube must be increased, which causes a decrease in heat transfer efficiency and an increase in thermal stresses generated due to the temperature difference between the outer and inner surfaces.
Accordingly, an object of the present invention is to solve the above problems and to provide a carbon deposition preventive tube for handling carbonaceous compounds such as hydrocarbons, their derivatives, carbon monoxide, or the like at temperatures higher than about 500.degree. C., which can also be thin walled with high strength.